1. Field of the Invention
The present invention relates to a radiographic intensifying screen (hereinafter referred to as "intensifying screen"). More particularly, the present invention relates to an intensifying screen excellent in durability.
2. Discussion of Background
An intensifying screen is used in intimate contact with an X-ray film in order to improve sensitivity of photographing in the field of medical radiographing for medical diagnosis or of industrial radiographing for non-destructive inspection of materials. Generally, on the surface of the intensifying screen, there are made abrasions or defects by an X-ray film, or dirt is attached thereon. Also, the surface of the intensifying screen is often damaged by contaminants including dust entered between the intensifying screen and the X-ray film, and also chemical materials contained in cleaners for the intensifying screen and the X-ray film are sometimes invaded into the intensifying screen to stain or color the screen. The above-mentioned various defects and damages cause unusual artifacts on a radiograph or make sensitivity lower. In order to prevent the performance of the intensifying screen from deteriorating, it is usual to provide a transparent protective layer on the surface of the intensifying screen which is brought into direct contact with an X-ray film.
Heretofore, in a method for forming a protective layer, a protective layer-forming coating solution having an appropriate viscosity is prepared by dissolving cellulose derivatives such as cellulose acetate, nitro cellulose and cellulose acetate butyrate, polyvinyl chloride, polyvinyl acetate, vinyl chloride-vinyl acetate copolymer, polycarbonate, polyvinyl butyral, polymethyl methacrylate, polyvinyl formal, polyurethane or other resins in a solvent, and the coating solution thus prepared is coated on a previously formed fluorescent layer and dried to form a protective layer thereon. Alternatively, a protective layer previously formed in the form of a film, such as an organic macromolecule film including polyethylene terephthalate, polyethylene, polyvinylidene chloride, polyamide and the like, may be laminated on a fluorescent layer to form a protective layer.
It is useful for improving durability of an intensifying screen to make a protective layer thick, but if the thickness of the protective layer increases, sharpness is lowered, and therefore it has been difficult to improve both durability and image quality at the same time.
As a method for improving durability and handling property of an intensifying screen or a radiation image-conversion panel using an photostimulable phosphor, Japanese Unexamined Patent Publication No. 310900/1992, Japanese Unexamined Patent Publication No. 309898/1992 and Japanese Unexamined patent Publication No. 75097/1994 disclose a protective layer formed on the surface of a fluorescent layer by coating a protective layer-forming coating solution containing an organic solvent-soluble fluorocarbon resin having a polysiloxane-structured oligomer, a perfluoroalkyl group-containing oligomer, a perfluoroolefin resin powder or a silicone resin powder added therein.
Among these protective layer-forming methods, when a coating solution prepared by dissolving a protective layer-forming resin in a solvent is coated on a fluorescent layer, a part of the coating solution is soaked into the inside of the fluorescent layer and accordingly a protective layer is formed on the fluorescent layer without making a boundary between the two layers. Thus, the protective layer is firmly bonded with the fluorescent layer, and peeling of the protective layer off the intensifying screen and occurrence of pinholes on the protective layer due to the presence of contaminants can be avoided. Also, when the above-mentioned organic solvent-soluble fluorocarbon resin is used as a protective layer-forming resin, anti-fouling property is improved and a coefficient of friction is lowered, thereby improving durability resistance. Further, since a contact angle between water and the resin is large, even if pinholes are produced, a chemical material from an X-ray film is hardly soaked and spot-like sensitivity degradation does not substantially occur, thus improving pinhole resistance.
However, when a protective layer is formed by coating a solution, a starting material used is limited to a solvent-soluble resin, and accordingly durability resistance is poor as compared with a method wherein an organic macromolecule film such as polyethylene terephthalate is laminated on a fluorescent layer to form a protective layer. Further, when a binder resin content in a fluorescent layer is reduced in order to improve sharpness, a protective layer-forming coating solution is soaked into the fluorescent layer when the protective layer-forming coating solution is coated on the fluorescent layer, and a protective layer having a sufficient thickness can not be formed. On the other hand, when a protective layer-forming coating solution is coated in a large amount on a fluorescent layer in order to form a protective layer having a sufficient thickness, the protective layer-forming coating solution is soaked into the fluorescent layer, thereby causing such problems as lowering sharpness or generating foams during coating.
Unlike the method for forming a protective layer by coating a solution, in the method for forming a protective layer by laminating an organic macromolecule film on a fluorescent layer, there is caused no problem of soaking with a protective layer-forming coating solution. Particularly when a polyethylene terephthalate film is used as a protective layer to be laminated, as compared with the method of using a protective layer-forming coating solution, abrasion resistance and solvent resistance are excellent and water vapor permeability and gas permeability are low, thereby providing excellent anti-staining property to a chemical material eluded from an X-ray film. However, as compared with a protective layer formed by coating a solution, adhesive strength of a protective layer laminated on a fluorescent layer is poor and therefore the laminated protective layer is liable to be peeled and pinholes are liable to occur when contaminants invade into between an intensifying screen and an X-ray film. Further, through the pinholes, various contaminants invade into the intensifying screen, thereby causing a problem of producing spot-like sensitivity degradation parts.
Thus, both a protective layer formed by coating a solution on a fluorescent layer and a protective layer formed by laminating an organic macromolecule film on a fluorescent layer respectively provide various advantages and disadvantages, and it has been difficult to satisfy all of requirements.
Also, recently, radiographing is automatically conducted in a labor saving manner, and an X-ray film is automatically conveyed and charged into a radiographing apparatus. Further, a film changer for automatically taking an X-ray film after radiographing and a film-conveying apparatus of a cassetteless X-ray TV are often used. Under these recent circumstances, an intensifying screen is demanded to be more improved in respect of anti-staining property, handling properties including an X-ray film-conveying property, and the like.
An object of the present invention is to provide an intensifying screen which satisfies satisfactory image quality, durability and handling performances at the same time.
In order to improve durability and handling performances of an intensifying screen without degrading image quality, the present inventors have studied about materials used for a protective layer of an intensifying screen and its structure, and have found that the material quality and the structure of the protective layer are closely related to durability and handling performances of the intensifying screen. The present invention is made on the basis of this finding.